Millimeter wave imaging systems have been widely used. For example, such systems have been used for security reasons such as detecting concealed weapons and obstruction under low visibility conditions. Many current airport scanners perform holographic reconstruction of a target object, but such systems require rotationally scanning a detection arm, which is time consuming. Alternatives, such as focal plane array (FPA) imaging, allow for both passive and active techniques but requires a large array of detectors for high resolution and quality.
State of the art time of flight imaging can performed with focused/collimated beams (time of flight information is obtained along one cross-range ray) either in the receive or the illumination arms, or both. Other imaging techniques may use diverging beams that rely on a large number of spatial samples (field of view requirement) in a large aperture (cross-range resolution requirement). Radar imaging and synthetic radar imaging (SAR) reconstruction algorithms assume that the object is a volume in three dimensional space. This assumption facilitates image reconstruction with only a few measurements.
Surface imaging techniques, such as inverse scattering techniques, and algorithms with coherent diverging beams make assumptions about the electrical boundary of the object as required by the electromagnetic models and also require exhaustive illumination and detection views. This technique uses a sparse measurement scheme and does not make any assumptions about the electrical boundary of the object, the only assumption is that the object can be approximated by a surface in three dimensional space.
Although significant advancements have been made in imaging systems and techniques, there is a continuing need for improved systems and techniques.